CRTP compiling error

Question

The following will compile with GCC 5.2 but not with Visual Studio 2015.

template 
struct CRTP {
    static constexpr int num = Deriv         


        

Answer 1

The problem is here:

template 
struct CRTP {
    static constexpr int num = Derived::value + 1;
                               ↑↑↑↑↑↑↑↑↑
};

At the time of the instantiation of CRTP, A isn't a complete class yet, so you can't actually access static members of it.

One workaround is to pass in num as a separate template argument:

template 
struct CRTP {
    static constexpr int num = N;
};

struct A : CRTP {

};